Fluid Ejections in Nature
Unifying fluid excretions from cicadas to elephants
A world of fluid excretion. Figure from our papers, linked below.
how cicadas redefine our understanding of fluid dynamics
From whale blowholes to human sneezes, fluid ejections are ubiquitous throughout nature and are utilized in multiple biological processes, including eating and excreting. The cicada is one creature whose small size means that excreting fluid comes with special physical challenges. But the cicada doesn’t let this stop it; cicadas are capable of excreting high-speed jets abnormal for their body size. Cicada jet urine can have a velocity of up to 3 meters per second— the fastest of all the animals assessed in our research, including mammals like elephants and horses. What insights can the urinary behaviors of cicadas offer into the principles of fluid dynamics across scales?
The characteristics of fluid waste ejection—whether it manifests as dripping or jetting—are governed by scale (size), inertia (speed), geometry (shape), and surface tension. While the importance of feeding for the growth and survival of animals is widely recognized, the waste elimination process is unexplored. This is despite feeding and excretion being obligatory hallmarks of all living systems.
“Cicada urination stays in quite a unique region in fluid dynamics, where both inertia and capillary forces play significant roles simultaneously over gravity”
Vampire bat peeing in droplets. National Geographic.
Our study specifically challenges the mammal-centric fluid excretion model that suggests jetting is exclusive to animals weighing over 3 kg due to large influence of surface tension and viscous forces at small scales (Yang et al., PNAS 2014). Like the tiny bat above, there is more to fluid ejection than previously thought. Forget what you know about mammals and insects— as we peer into the pee world, we’ll find something un-pee-lievable.
“Biology and the diversity across forms of life has the potential to drive advances in fields from healthcare to manufacturing and other key industries. This work shows that even the way in which organisms get rid of waste can provide new insights into fluid dynamics that spur innovation in soft robotics and ways to handle fluid at small scales in all manner of manufacturing. It also shows how much we still have to learn about organisms like cicadas, that are often covered in the news.”
Cobra spitting venom. NHM Images.
This project has been featured in New york times, popular science, scientific american and more
Major questions
How do cicadas eject fluid jets?
How do cicada jets challenge what we know about fluid excretion across other taxa?
How we can apply this to other fluid transport in nature - beyond urination?
What we’ve discovered
In a Tiny World, Surface Tension Reigns Supreme. Cicadas have a solution.
At small scales, for tiny insects and mammals, droplets adhere to surfaces due to surface tension, making getting rid of fluid an extra challenge. This often requires innovative strategies to overcome the obstacle of surface tension (e.g., droplet catapulting in sharpshooter insects, kicking in aphids, maternal licking in mice, or hydrophobic coating in gall aphids). But compared to other xylem-sap feeders, cicadas’ relatively large body and orifice size makes jetting energetically and mechanically feasible. With their jets, cicadas can quickly process large volumes of nutritionally poor food and places them a the smallest known animals to form high-speed jets in a surface tension-dominated regime (Bo < 1).
We Created a Unifying Framework of Fluid Ejection
Across taxa and physical scales, organisms employ various fluidic waste release strategies, quantifiable on a We − Bo framework. Large mammals occupy the Bo > 1, We > 1 quadrant, where inertial forces dominate, driven by bladder pressure and gravitational forces. At extremely high values (Bo >> 1, We >> 1), fluids may exit as sheets, as in elephants and cows, where gravity drives inertial flows. Invertebrates like cicadas, bumblebees, and butterflies, characterized by Bo < 1, We > 1, demonstrate high-speed inertial jets for excretion, differing significantly from similarly sized mammals’ strategies (e.g., bats, mice). Cicadas have the smallest orifice diameter to form fluidic jets, despite being in a surface tension-dominant regime. In comparison, slightly larger bats and mice excrete in droplets on the same x-axis (Bo), further underscoring the unusual high-speed jet ejection in tiny cicadas. No organisms have been identified in the Bo > 1, We < 1 quadrant, a quadrant where the organisms would exclusively rely on gravitational forces for droplet-based excreta removal.
Beyond Urination- Complex Fluid Ejections in Nature
Fluid transport encompasses a world beyond urination. It ranges from viscoelastic fluids like velvet worm slime jets, rapid propulsion like octopus jetting or microvelia’s marangoni propulsion, precise shooting of archerfish water jets, or even explosive fluid expulsion like human sneezes and dolphin blowholes. The transition to a more complex understanding of fluid ejections, beyond the Newtonian framework, holds significant relevance for real-world applications.
Technology Inspired by Natural Fluidic Ejections
Drawing inspiration from natural systems has already led to innovations such as aircraft and aerial robot wing designs, gecko-inspired adhesive materials, and superhydrophobic structures. Similarly, fluid ejection systems in living organisms presents a new frontier for bio-inspired design, such as underwater robot propulsion, drug delivery, or nozzles for printing.
Read the papers
Unifying Fluidic Excretion Across Life from Cicadas to Elephants, PNAS (2024)
Fluid ejections in nature, Annual Reviews of Chemical & Biomolecular Engineering (2024)